【HDOJ】4612 Warm up

双连通缩点+求树的直径，图论基础题目。

/* 4612 */
#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <cstdio>
#include <cstring>
#include <cstdlib>
using namespace std;

#define MAXV 200005
#define MAXE 1000005

typedef struct {
    int v, next;
} Edge_t;

Edge_t E[MAXE*2];
vector<int> vc[MAXV];
int low[MAXV], pre[MAXV];
int bn[MAXV];
int S[MAXV], top;
int head[MAXV], L;
bool visit[MAXV];
int dfs_clock, block;
int n, m;

void addEdge(int u, int v) {
    E[L].v = v;
    E[L].next = head[u];
    head[u] = L++;
}

void init() {
    memset(head, -1, sizeof(int)*(n+1));
    memset(pre, 0, sizeof(int)*(n+1));
    memset(bn, 0, sizeof(int)*(n+1));
    dfs_clock = block = L = top = 0;
}

void tarjan(int u, int fa) {
    int i, j, k, v;
    
    S[top++] = u;
    pre[u] = low[u] = ++dfs_clock;
    for (i=head[u]; i!=-1; i=E[i].next) {
        if (i == (fa^1))
            continue;
        v = E[i].v;
        if (!pre[v]) {
            tarjan(v, i);
            low[u] = min(low[u], low[v]);
        } else if (!bn[v]) {
            low[u] = min(low[u], pre[v]);
        }
    }
    if (low[u] == pre[u]) {
        ++block;
        do {
            bn[S[--top]] = block;
        } while (S[top]!=u);
    }
}

void solve() {
    queue<int> Q;
    int i, j, k;
    int u = 1, v;
    
    memset(pre, 0, sizeof(int)*(block+1));
    pre[u] = 1;
    Q.push(u);
    while (!Q.empty()) {
        u = Q.front();
        Q.pop();
        for (i=0; i<vc[u].size(); ++i) {
            v = vc[u][i];
            if (!pre[v]) {
                pre[v] = 1;
                Q.push(v);
            }
        }
    }
    
    memset(pre, 0, sizeof(int)*(block+1));
    pre[u] = 1;
    Q.push(u);
    while (!Q.empty()) {
        u = Q.front();
        Q.pop();
        k = pre[u];
        for (i=0; i<vc[u].size(); ++i) {
            v = vc[u][i];
            if (!pre[v]) {
                pre[v] = k+1;
                Q.push(v);
            }
        }
    }
    
    printf("%d
", block-pre[u]);
}

int main() {
    int i, j, k;
    
    #ifndef ONLINE_JUDGE
        freopen("data.in", "r", stdin);
        freopen("data.out", "w", stdout);
    #endif
    
    while (scanf("%d %d",&n,&m)!=EOF && (n||m)) {
        init();
        for (i=0; i<m; ++i) {
            scanf("%d %d", &j, &k);
            addEdge(j, k);
            addEdge(k, j);
        }
        for (i=1; i<=n; ++i)
            if (!pre[i])
                tarjan(i, -1);
        for (i=1; i<=block; ++i)
            vc[i].clear();
        for (i=1; i<=n; ++i) {
            for (j=head[i]; j!=-1; j=E[j].next) {
                k = E[j].v;
                if (bn[i] != bn[k]) {
                    // bridge
                    vc[bn[i]].push_back(bn[k]);
                    vc[bn[k]].push_back(bn[i]);
                }
            }
        }
        solve();
    }
    
    return 0;
}